Flywheels for prime movers, or combustion engines, are known. A ring gear is often fixed to the flywheel to aid in starting the combustion engine. A starter includes a gear for engaging with the ring gear to spin the flywheel and rotate the engine. This begins the combustion process.
It is known to attach a ring gear to a flywheel by press-fitting or shrink fitting the gear onto a cylindrical ring of the flywheel. In either case, an inner diameter of the ring gear is slightly smaller than an outer diameter of the flywheel ring. For press-fitting, the smaller ring gear inner diameter is forced under load onto the larger flywheel outer diameter. Shrink fitting eases this process by heating the ring gear to increase the inner diameter and/or cooling the flywheel to reduce the outer diameter so that less load or even no load is required to assemble the components.
Testing under extreme maneuvers, or extreme operating conditions, such as high heat and/or high speed can expand the ring gear such that it can loosen on the flywheel. For example, a slipping clutch can generate excess heat, reducing clutch torque capacity allowing the engine to “run away” or reach high rotational speeds. In extreme cases, testing has shown that some ring gears can loosen on the flywheel causing internal damage to the engine or transmission. In exceptional cases, a dislodged ring gear may even injure vehicle passengers.